It is known that the handling of a vehicle can be improved by steering not only the front wheels but also the rear wheels according to the vehicle speed, in particularly by steering the rear wheels in an opposite phase relationship relative to the front wheels in low speed range and in a same phase relationship relative to the front wheels in high speed range.
U.S. Pat. No. 4,572,316 discloses a four-wheel steering system in which the rear wheels are actuated by a hydraulic cylinder. The hydraulic cylinder is in turn controlled by a valve which is controlled according to the steer angle of the front wheels and the vehicle speed. However, this prior art system has the disadvantage of the complexity of the control system. It is also a disadvantage that the rear wheels are steered by the hydraulic cylinder without any direct linkage between the steering wheel and the rear wheels. U.S. Pat. No. 4,705,135 discloses a four wheel steering system responsive to a vehicle speed in which a mechanical coupling is used between the steering wheel and the rear wheels. A motor driven mechanical actuator is provided for varying the steering angle ratio of the rear wheels to the front wheels as a continuous mathematical function of the vehicle speed. However, the mechanism for varying the steering angle ratio of the rear wheels to the front wheels is relatively complicated and large in size, and may not be suitable for certain practical applications where little space is available for such a mechanism.